An equalizer is an electronic device that attempts to reverse distortion incurred when a signal is transmitted through a channel. In digital communications, an equalizer promotes reduction of inter-symbol interference. Distortion occurs because a channel's frequency response is not perfectly linear. If an equalizer's transfer function is the inverse of the channel's transfer function within the frequency range of the transmitted information, the equalizer can provide a flat response. The transfer function of an equalizer is commonly defined by circuitry having a number of nodes or taps that correspond to coefficients of the transfer function. Each tap has one or more controls, such as gain and dither. Setting the gain, dither or other tap controls establishes the transfer function.
In some equalizers, the tap controls or settings are fixed or hard-wired into the circuitry. In other equalizers, a user can program these settings. In still other equalizers, known as adaptive equalizers, tap control signals are generated in response to inputs that can vary. For example, an adaptive equalizer can vary the tap control signals in response to mean squared error (MSE) in a feedback-based manner. MSE is a measure of the difference between the transmitted data and the equalizer output (i.e., the equalized signal). An adaptive equalizer attempts to determine the set or group of tap settings that results in the least MSE, i.e., minimization of the MSE. Some adaptive equalizers perform such a process during a training sequence in preparation for data transmission. Other adaptive equalizers perform such a process dynamically during the data transmission. The latter type of adaptive equalizer may perform an iterative process that converges to a solution that minimizes the MSE.
Adaptive equalizer circuitry can include Decision Feedback Equalizer (DFE) circuitry, Feedforward Equalizer (FFE) circuitry, continuous-time linear equalizer (CTLE) circuitry, or a combination of one or more of these. Some adaptive equalizers are capable of adapting to changes in the data rate of the signal being transmitted over the channel. Such adaptive equalizers are known as rate-adaptive equalizers. One type of rate-adaptive equalizer uses programmable delay cells to adjust the delay of the equalizer based on the change in the data rate. Such rate-adaptive equalizers are limited in terms of dynamic range, precision and power consumption. Another type of rate-adaptive equalizer uses a clock-and-data recovery (CDR) circuit to lock onto the frequency of the data signal to recover a clock rate and then controls clocked delay elements (i.e. flip flops) of the equalizer in accordance with the recovered clock rate.
Rate-adaptive equalizers that use CDRs in this manner are more precise and power-efficient than those that use programmable delay cells. CDR-based rate-adaptive equalizers, however, have limitations in terms of initializing the tap settings, i.e., the coefficients of the transfer functions of the FFE/CTLE and the DFE of the equalizer. The equalizer needs to equalize the input data stream in order to allow the CDR to lock onto the incoming data frequency. If the CDR is not locked, the operation of the clocked delay cells will not be optimal and will therefore lead to errors. The equalizer, however, cannot properly perform equalization without the CDR being locked to the incoming data frequency. This presents somewhat of a problem when it comes to initializing the tap settings.
One solution to overcoming this problem is to have a user manually enter the initial tap settings. This is typically accomplished by a user who enters the initial tap settings into a register file of an IC digital interface such as I2C to the system controller IC. In order to choose the correct initial tap settings, however, the user must have certain knowledge about the communication channel, which the user may not possess. To further complicate matters, channel conditions may change over time, which would require a change to the initial tap settings. A customer who purchases equipment containing the equalizer may not have sufficient knowledge to correctly choose the tap settings.
Accordingly, a need exists for a rate-adaptive equalizer that is capable of self-initialization, thereby eliminating the need for a user to be involved in initializing the equalizer with the correct tap settings.